The invention relates to a light management film. In a preferred form, the invention relates to a light management film with a layer capable of receiving a colorant density of at least 0.5 useful for use in a backlit display application or an overhead projection system.
Current printing operations rely on flat media in which to print on. Thermal media producers go to great lengths to ensure that the thermal print media is flat. Thermal media needs to be flat to ensure even color density printing because the print head and paper must hit at the same point. As disclosed in U.S. Pat. No. 5,244,861, the thermal print media must have an average surface roughness of less than 0.2 micrometers or else there will be dropout. Dropout occurs when there are valleys or peaks in the thermal print media and the print head (because the thermal media is then closer or farther from the print head) so that a density differential occurs. If there is a valley in the thermal paper, less colorant will be transferred and it will show up as a light colored spot on the image.
Current light management films work by incorporating light shaping elements and typically have a surface roughness over 3 micrometers. It has been difficult to print over these light-shaping films to add color of high density.
Thermal printing is a term broadly used to describe several different families of technology for making an image on a substrate. Those technologies include hot stamping, direct thermal printing, dye diffusion printing and thermal dye sublimation printing. In order to use thermal dye sublimation printing on a non-compatible surface, the most common methods of improving print quality is to increase the thermal energy of the print head and to increase the pressure applied to the print head by the backup roll. However, increasing thermal energy and pressure can lead to decreased printer head life, ribbon wrinkling, lower print quality and mechanical stresses in the printing system.
It would be desirable to have a light management film that has excellent light shaping characteristics and is able to reproduce highly saturated colors. It would also be desirable to be able to color to monochromatic displays, such as LCD""s for cell phones and personal digital assistants (PDAs) to create interesting static displays with coloration or personalization.
Prior art diffuser films with uniform diffusion across the film must have the diffusion efficiency to diffuse the most intense, specular areas of the display across the entire diffuser leading to high levels of diffusion efficiency causing lower percentages total transmission across the entire film. It would be desirable to have the amount of diffusion, light shaping, and color vary across the diffusion film, whether a bulk or surface diffuser, to compensate for uneven brightness across a backlit display.
Current overhead projection media typically consists of a transparent film with printing of some color and density. The projection film is placed on a light source where light is transmitted through the projection film and collected by the collection lens. The collection lens then projects the light to a projection screen. The projected image is much larger than the original projection film, making this system suitable for multiple viewers, as in a conference room or a classroom.
Because the bright white background of the projected image is bright, the viewers cannot easily view light colors printed on the transparency. In addition to the light coming from the overhead projector and comprising the image information, ambient light will be incident on the projection screen and reflected by this screen towards the audience. To obtain an image on the projection screen that is sufficiently rich in contrast during a presentation, at which illustrations, graphs and the like are shown by means of an overhead projector, the audience space will have to be darkened considerably. An increase of the power of the lamp in the illumination system, so that the signal light will be considerably more intense than the ambient light, causes heat build up in the lamp with causes early failure of the bulb or installation of a noisy fan. It would be advantageous to have a transparency film that could be projected with bright and dark areas with colors.
The current barcode system has a number of drawbacks. The typical black and white barcodes take up valuable packaging space and are unattractive. Where a packaging label is small (on a pack of chewing gum for example) the barcode might take up half of the label. Because the barcode takes of space and is unattractive, it is typically only placed in one part of the package. The operator of the barcode scanning system, for example a cashier at a grocery store, has to know where the barcode is on each package to quickly scan the item. If it is a new package design or a new item, the cashier might have to turn the package over a few times to find the barcode. It would be desirable to have a barcode system that could be placed in multiple areas of the package, without taking up addition space of the label, so it is more easily scanned.
Light diffusing elements that scatter or diffuse light generally function in one of two ways: (a) as a surface light diffusing element utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk light diffusing element flat outer surfaces and embedded light-scattering elements. The surface light diffusing elements normally utilize the rough surface, typically with a lens, exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. The bulk diffuser diffuses the light within the film. Examples are small particles, spheres, or air voids of a particular refractive index are embedded another material with a differing refractive index. Light shaping elements tend to direct or deflect light using geometries and an index of refraction change. A prism is one example of a light shaping clement. Light shaping elements tend to be surface light shaping elements, though there are a few bulk light shaping elements. The geometry, materials, and environment determine how much light is shaped or directed by a light shaping element.
U.S. Pat. No. 4,774,224 describes using a resin-coated paper with a surface roughness measurement of 0.19 micrometers or less. This type of paper is generally used for photographic bases, and consequently, it has the photographic look. This base has excellent curl properties both before and after printing, and due to its simple design is relatively inexpensive to manufacture. However, it is not very conformable and under printing conditions with low pressure between a print head and a printer drum, it does not yield high uniformity prints (most commercial printers are now being built with low printing pressures to make them more cost effective). Also higher energy levels are needed to achieve a given density. It has been shown that typical resin coated paper cannot achieve high density because polyolefin, the polymer typically used in resin-coated paper, is not as good a thermal dye sublimation receiver as other polymers.
In U.S. Pat. No. 6,381,068 (Harada et al.), the diffusing element may be a bulk diffuser including a transparent base material and at least one light-diffusing material, such as a pigment and/or beads, dispersed in the transparent base material. The pigments used may include a white pigment (for example, titanium oxide) and may also include one or more colored pigments. The pigments in this invention are only used with a uniform diffuser and not a variable diffuser. Furthermore, the colored pigments must be a single color tone and density across the display.
In U.S. Pat. No. 6,266,476 (Shie et al.), a monolithic element comprises a substrate body and a macro-optical characteristic produced by surface micro-structures. These micro-structures can be non-uniform across the lens to minimize certain lens aberrations. These non-uniform micro-structures reduce lens aberrations, but are not able to significantly alter the macro-optical characteristics of the optical body. The diffusing structures, in this invention, vary so as to change the macro diffusion efficiency across the diffusion film.
U.S. Pat. No. 5,852,514 (Toshima et al.) describes a light diffusing element comprising a light diffusion layer including acrylic resin and spherical particles of polymethyl methacrylate on a transparent support. Whereas this film would diffuse the light efficiently, the polymers used have high glass transmission temperatures, and it would therefore be difficult to melt the spherical particles completely to create areas of specular transmission. When projected these not completely melted lenses would diffuse a portion of the light lowering the brightness of the printed, more specular, projected areas and thus lower contrast of the overhead projected image.
U.S. Pat. No. 3,763,779 (Plovan) discloses a method for copying an image by selectively coalescencing microporous voids in a voided film to create areas of transparency. The method has limitations in that to produce a copy in the voided film, an original must be used and the original must be of a particular material and format. It would be desirable to have a process to selectively coalesce voids using an electronic file as the template. The film has voids throughout the thickness of the film so that to make an area of the film transparent, the voids throughout the thickness of the film must all be coalesced or melted. This requires a substantial amount of energy making this method expensive, time consuming, and difficult.
U.S. Pat. No. 6,386,699 (Ylitalo et al.) discloses an embossed media for use as an inkjet receiver. This receiver could be part of a transparency media for overhead projection. The embossed surface is used to catch the inkjet materials and allow for drying time. The inkjet process does not change the structure of the embossed surface of the receiver media and would therefore transmit the projected image as all the same light intensity on the screen when lit by the overhead projection system.
U.S. Pat. No. 4,497,860 (Brady) discloses a method of using a linear prism array to project areas onto an overhead projection screen of high and low illumination. The method involves using a sheet with an ordered linear prism array on one side and heat to create non-refracting areas that show up as bright areas when projected. The light that passes through the still intact linear prism array is deflected away from the collection lens and is viewed as darker areas of illumination on the screen. By adding a diffraction grating and another linear prism array, the method can create colored images where the projected image can have at most two colors (one non-refracting area color and one refracting area color), for example, purple text on a green background. It would be desirable if more than two colors could be displayed by the overhead projection system at once. An undesirable aspect is that the process of molding the linear prism array must be very exact, leading the method to become cost prohibitive. A small difference in molding temperature or time yields vastly different qualities of projection media. In addition, when then linear prisms are melted, it the melting tool is not correctly aligned with the linear prism array, then some of the prisms will be melted partially distorting the projected image in color and density because the half-melted lens array will not deflect light correctly. A moirxc3xa9 pattern may be visible when the films are projected because of the use of an ordered surface pattern, especially when the one linear prism array is placed in conjunction with another linear prism array or diffraction grating. This moirxc3xa9 pattern is undesirable as it detracts from the projected material.
U.S. Pat. No. 5,369,419 (Stephenson et al.) describes a thermal printing system where the amount of gloss on a media can be altered. The method uses heat to change the surface properties of gelatin, which has many disadvantages. Gelatin can not achieve high roughness averages, thereby having a low distinction between the matte and glossy areas of the media. This small distinction between the matte and glossy states lead to a low signal to noise ratio and when projected, creates lower contrast ratios. Gelatin also is very delicate, scratch prone, is self-healing, tends to flow over time thus changing its surface roughness and other properties time especially in high humidity and heat, and is dissolved if placed in water. Also, gelatin has a native yellow color, is expensive, and is tacky sticking to other sheets and itself. It would be desirable to use a material that had no coloration, is more stable in environmental conditions, and could have a higher surface roughness.
U.S. Pat. No. 2,739,909 relates to a heat-sensitive recording paper by overcoating black-colored paper with a continuous thermoplastic resin material containing microscopic voids dispersed throughout the resin. The coating layer is opaque, but becomes transparent by the localized action of a stylus using either heat or pressure or both to disclose the black color of the support. There is a problem with this element in that the manner of obtaining the voids is complicated which involves carefully controlled drying conditions of emulsions leading to low yields and expensive end products.
U.S. Pat. No. 5,818,492 (Look) and U.S. Pat. No. 5,508,105 (Orensteen et al.) teach that thermal mass transfer printing can be performed on retroreflective sheeting in those instances where there is a polymeric layer or layers disposed thereon. While adding a polymeric layer has improved printability on some retroreflective sheeting, the process of adding the layer increases the cost of the final product and can degrade the retroreflective properties of the substrate. Even with the additional layer, the print quality is inadequate for some graphics applications. Adding a printable layer may alter other characteristics of the retroreflective sheeting, such as frangibility.
U.S. Pat. No. 6,246,428 (Look) describes a method and apparatus for preheating the dye receiving layer to a certain temperature, in order to increase the thermal energy of the substrate surface to improve print quality at low print head thermal energy and pressure in a thermal mass transfer printing system. The method is suitable for webs that have a non-planar printing surface, such as unsealed retroreflective sheeting, and non-homogeneous thermal conductive, such as a seal or unsealed retroreflective sheeting. This method of printing light management films requires extra heat in the thermal printer (one or more depending on the configuration) leading to higher cost and energy usage. Furthermore, the large installed base of the thermal printers would need to be retrofitted, which would be cost prohibitive. It would be desirable to print light management films without changing the thermal printer configuration.
U.S. Pat. Nos. 5,302,574 (Lawerance et al.) and 5,387,571 (Daly) describe thermal dye receiving layers that can achieve dye densities of over 0.5, but the dye receiving layer is smooth having virtually no light management characteristics.
There remains a need for an improved light management film to provide light shaping characteristics and high color replication.
The invention provides a light management film containing a substrate, light shaping elements with a roughness average of at least 3 micrometers, and a layer capable of receiving a colorant density of at least 0.5.
The invention provides improved light management media to shape light as well as have high saturated color.